Try just -port=randomnumber as a command line switch

My coworker and I commonly had to do that with Arma2. We game a lot from the same network here, and a few games have a similar problem.

My understanding is that it's a flaw in the network architecture, possibly combined with a flaw in the way the router handles sessions. Not going to get it with all games or routers but the wrong combo will leave you screwed unless you can force one of the clients to a new port.

I have no idea if the above switch works in SA -- we haven't had to use it. But it did the trick in Arma2.